1. Field of the Invention
The present invention is in the field of inertial reference systems for use on moving vehicles and more specifically relates to a short-term strapdown inertial attitude and heading reference system which is assisted by updating inputs supplied by auxiliary instruments in the vehicle.
2. Description of the Prior Art
Inertial navigation systems have reached a relatively high state of development, characterized by very high accuracy and a relatively high cost. The attitude and heading reference system of the present invention is intended for use in situations where extreme accuracy over long periods of time is not required and where relatively low cost of acquisition and maintenance is desired. Nevertheless, the present invention is not merely a degraded inertial navigation system, but instead was conceived and developed from its inception as a low-cost short-term inertial reference system. Consequently, the present invention differs in its design from known inertial navigation systems in several important structural aspects, as will now be described.
Most prior art inertial systems include a mechanical element, such as a gyro or a stabilized platform which is used as an inertial reference element to "remember" a particular orientation fixed in inertial space, i.e., subject to no sensible translational or rotational accelerations. Owing to the motion of the vehicle relative to this inertial reference element, it was necessary in prior art systems to provide a gimbal system for mounting the inertial element to the vehicle, which necessitated the use of gimbal torquers, gimbal angle pickoffs, and other associated paraphernalia. The resulting inertial system tended toward mechanical complexity, heavy weight, high cost, reduced reliability and suceptibility to certain kinds of errors when accelerated or vibrated.
In contrast, the present invention can be described as a pseudo-inertial system because no mechanical component is used as an inertial reference element. The inertial reference is stored within the system as an electrical signal. This electrical signal, in various embodiments, may comprise an analog or digital representation of the inertial reference and may be stored in the memory of a computer. This has the advantage that mechanical complexity is reduced, resulting in lower weight, greater ruggedness, and lower cost.
Known attitude and heading reference systems of the prior art employ gyros to sense the rate of change of the vehicle's attitude. Although the construction of rate gyros is a highly developed art, the mean time before failure of the prior art systems tended to be limited by the life of the rotating components. Further, rate gyros are subject to errors induced by acceleration or vibration.
In contrast, the attitude and heading reference system of the present invention does not use rotating components at all. According to the present invention, angular rates are determined by mathematical integration of the output of angular accelerometers. Because the present invention contains no rotating components, it is insensitive to acceleration-induced errors, and has a relatively longer mean time before failure than prior art systems, resulting in lower maintenance costs.
The sensors used in some prior art systems had a rather limited operating range. There was a possibility that this range would be exceeded during certain maneuvers of the vehicle. When this happened, the output of the sensor was erroneous and misleading to the system. It therefore became common to provide auxiliary means associated with the sensor to disconnect it from the system when its operating range was exceeded. In addition to making the system more complicated by the addition of more parts and modes of operation, the "cut-out" technique resulted in reduced accuracy because of the loss of input data during those times when such data was most needed.
In contrast, the present invention avoids the "cut-out" by using, as its sensors, linear and angular accelerometers which are operable over a very wide range of accelerations and angular rates, and which are capable of operating continuously at all vehicle attitudes.
Unlike known prior art attitude and heading reference systems, the present invention includes means for calculating a sideslip angle, which is a measure of the sideward ground speed of the vehicle relative to its longitudinal speed. This output is of considerable usefulness if the vehicle is an aircraft making a landing approach under the control of an automatic landing controller.
The attitude and heading reference system of the present invention includes in combination a number of highly worthwhile features which will be described immediately below, and which further distinguish the present invention from the prior art.